Protein having xylose isomerase activity and use of same

ABSTRACT

A eukaryotic cell having xylose utilization ability. Provided is a protein that has xylose isomerase activity and has an amino acid sequence including, when aligned with an amino acid sequence expressed by SEQ ID NO:1, the 1st to 6th motifs expressed respectively by SEQ ID NOs:2 to 7 from the N-terminus side in the order described, and having, in place of asparagine (N) in an amino acid sequence of the 6th motif, another amino acid.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Division of application Ser. No. 14/780,987 filed Sep. 28, 2015, which in turn is a national stage entry of PCT/JP2014/001849 filed Mar. 28, 2014, which claims priority to JP 2013-070584 filed Mar. 28, 2013, and JP 2014-024878 filed Feb. 12, 2014. The disclosure of the prior applications is hereby incorporated by reference herein in its entirety.

The present application relates to a protein having novel xylose isomerase activity, and to a technique for producing a useful substance in use of this protein, with xylose being a carbon source.

TECHNICAL FIELD Background Art

Yeast, Saccharomyces cerevisiae, which is a fermentation microorganism for a production process of cellulose ethanol, is not able to utilize xylose included in vegetable biomass in a large amount. Therefore, researches for imparting xylose utilization ability to Saccharomyces cerevisiae are in progress. To this end, introduction of 2 types of pathways to the yeast is investigated. One is a pathway (XR-XDH pathway) using a xylose reductase (XR) and a xylitol dehydrogenase (XDH). However, there is a drawback in the pathway that intermediate metabolites accumulate and the ethanol yield decreases. Meanwhile, in the case of a pathway (XI pathway) using a xylose isomerase (XI), there is no such a drawback, but another drawback arises that the consumption rate of xylose is slow in comparison to the XR-XDH pathway. Therefore, various investigations are under way for a high performance XI.

Improvement of a XI originated from Piromyces sp. E2, which was reported as the first XI being able to function in yeast, has been carried out (Patent Literature 1, Non Patent Literature 1). Further, improvement of a XI originated from Ruminococcus flavefaciens has been also carried out (Patent Literature 2). Further, improvement of a XI originated from Lactococcus lactis has been also carried out (Patent Literature 3). Furthermore, a XI originated from an enteric protist of Reticulitermes speratus having higher xylose consuming ability of yeast compared to a heretofore known XI has been reported (Patent Literature 4).

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent Application Publication No.     2008-79564 -   [Patent Literature 2] WO 2011/150313 -   [Patent Literature 3] WO 2010/070549 -   [Patent Literature 4] Japanese Patent Application Publication No.     2011-147445

Non Patent Literature

-   [Non Patent Literature 1] Lee S, Jellison T, Alper H S. Appl Environ     Microbiol, 2012; 78 (16): 5708-16

SUMMARY OF INVENTION

However, the xylose consumption rate of an XI in Patent Literature 1 and Non Patent Literature 1 remained too low. Further, with respect to an XI in Patent Literature 1 the activity in yeast was not disclosed. Meanwhile, with respect to an XI in Patent Literature 2, although improvement of the growth rate of a transgenic yeast in a xylose culture medium has been recognized, the fermentation performance is not clear. Further, with respect to a XI in Patent Literature 3 the growth characteristic and fermentation performance in a xylose culture medium are not clear. Further, with respect to a XI described in Patent Literature 4, although XI ability in yeast has been improved, further improvement thereof is sought after.

Under such circumstances, an XI favorable for improving xylose consuming capacity and improving fermenting capacity in yeast has been still sought after.

According to the present description, a protein with XI activity useful for improving the xylose fermentation ability of yeast and a use of the same are provided.

Solution to Technical Problem

The inventors focused on an XI originated from an enteric protist in Reticulitermes speratus (hereinafter referred to as “RsXI”) and found that the xylose fermenting capacity of yeast could be improved by modification of the XI by introduction of a point mutation substituting another amino acid. Further, it was found that the amino acid substitution mutation introduced into the XI was also effective in another XI having a similar amino acid sequence. Based on the findings, the following means are disclosed hereunder.

[1] A protein that has xylose isomerase activity and has an amino acid sequence including, when aligned with an amino acid sequence expressed by SEQ ID NO:1, the following 1st to 6th motifs from the N-terminus of the protein in the order described, and having, in place of asparagine (N) in an amino acid sequence of the 6th motif, another amino acid: 1st motif: FXXXXKXXXXXXXXHDXD (SEQ ID NO:2)

wherein X represents a naturally occurring amino acid,

2nd motif: XXXXXXXWGGREGYXXLXNT (SEQ ID NO:3)

wherein X represents a naturally occurring amino acid,

3rd motif: XXXXXXXXEPKPXEPXXHQYDXD (SEQ ID NO:4)

wherein X represents a naturally occurring amino acid,

4th motif: LXXXXXXNXEXNHXXLXXHXXXH (SEQ ID NO:5)

wherein X represents a naturally occurring amino acid,

5th motif: XGSXDXNXGXXXXGWDXDXXP (SEQ ID NO:6)

wherein X represents a naturally occurring amino acid, and

6th motif: GGXNFDXKXRR (SEQ ID NO:7)

wherein X represents a naturally occurring amino acid.

[2] The protein according to [1], wherein:

the 1st motif is expressed by FXXXXKXGXXXXXFHDXD (SEQ ID NO:8),

the 2nd motif is expressed by XXXXXVFWGGREGYXXLLNT (SEQ ID NO:9),

the 3rd motif is expressed by XXXXXFXIEPKPXEPXXHQYDXD (SEQ ID NO:10),

the 4th motif is expressed by LXXXFKXNXEXNHXXLAGHXXXH (SEQ ID NO:11),

the 5th motif is expressed by XGSXDXNXGXXXXGWDTDXFP (SEQ ID NO:12), and

the 6th motif is expressed by GGXNFDXKXRR (SEQ ID NO:13).

[3] The protein according to claim [1] or [2], wherein:

the 1st motif is expressed by FEXXXKXGXXXXCFHDXD (SEQ ID NO:102),

(wherein position 3 is F or I or L; position 4 is A or M; position 5 is E or Q or S or T; position 7 is L or M; position 9 is I or V; position 10 is E or K or P; position 11 is F or Y; position 12 is F or Y; and position 17 is A or I or V),

the 2nd motif is expressed by GXXXYVFWGGREGYXXLLNT (SEQ ID NO:103),

(wherein, position 2 is A or G; position 3 is V or K or E; position 4 is G or N; position 15 is E or M; and position 16 is S or T),

the 3rd motif is expressed by XXXXXFXIEPKPXEPXXHQYDXD (SEQ ID NO:10),

(wherein, position 1 is G or N; position 2 is F or H; position 3 is K or D or L; position 4 is G or P; position 5 is D or T or I; position 7 is L or Y; position 13 is K or M; position 16 is M or T; position 17 is K or T; and position 22 is F or V),

the 4th motif is expressed by LXKXFKXNXEXNHAXLAGHTFXH (SEQ ID NO:104),

(wherein, position 2 is D or E; position 4 is D or Y; position 7 is L or M or V; position 9 is I or L; position 11 is A or T or V; position 15 is T or W; and position 22 is Q or E),

the 5th motif is expressed by XGSXDANXGXXXXGWDTDXFP (SEQ ID NO:105),

(wherein, position 1 is F or L; position 4 is I or V; position 8 is Q or R or T; position 10 is D or N; position 11 is P or Y; position 12 is L or N or Q; position 13 is L or N, and position 19 is E or Q), and

the 6th motif is expressed by GGXNFDXKXRR (SEQ ID NO:13),

(wherein, position 3 is I or L or T; position 7 is A or S; and position 9 is T or V).

[4] The protein according to any one of [1] to [3], comprising an amino acid selected from the group consisting of cysteine, threonine, valine, and alanine in place of asparagine (N) in the 6th motif

[5] The protein according to any one of [1] to [4], comprising threonine or cysteine in place of the asparagine.

[6] The protein according to any one of [1] to [5], wherein

the 1st motif is composed of an amino acid sequence having an identity of 65% or more with the amino acid sequence expressed by SEQ ID NO:24,

the 2nd motif is composed of an amino acid sequence having an identity of 75% or more with the amino acid sequence expressed by SEQ ID NO:25,

the 3rd motif is composed of an amino acid sequence having an identity of 65% or more with the amino acid sequence expressed by SEQ ID NO:26,

the 4th motif is composed of an amino acid sequence having an identity of 70% or more with the amino acid sequence expressed by SEQ ID NO:27,

the 5th motif is composed of an amino acid sequence having an identity of 70% or more with the amino acid sequence expressed by SEQ ID NO:28, and

the 6th motif is composed of an amino acid sequence having an identity of 70% or more with the amino acid sequence expressed by SEQ ID NO:29.

[7] A DNA coding for the protein according to any one of [1] to [6].

[8] A transformation vector for a eukaryotic cell, containing the DNA according to [7].

[9] A eukaryotic cell retaining the DNA according to [7].

[10] The eukaryotic cell according to [9], which is yeast.

[11] The eukaryotic cell according to [10], wherein the yeast belongs to any one selected from the group consisting of Saccharomyces, Kluyveromyces, Candida, Pichia, Schizosaccharomyces, Hancenula, Klocckera, Schwanniomyces, Yarrowia, and Issatchenkia.

[12] The eukaryotic cell according to any one of [9] to [11], which produces secretory cellulase.

[13] The eukaryotic cell according to any one of [9] to [12], having an exogenous or endogenous gene that produces any one selected from the group consisting of ethanol, lactic acid, acetic acid, 1,3-propanediol, propanol, butanol, succinic acid, ethylene, glycerol, farnesol, geranylgeraniol and squalene. [14] A method for generating a eukaryotic cell with imparted or improved xylose utilization ability, comprising a step of introducing the DNA according to [7] into a eukaryotic cell for transformation. [15] A method for producing a useful substance, comprising a step of culturing the eukaryotic cell according to any one of [9] to [13] in the presence of xylose. [16] The production method according to [15], wherein the useful substance is any one selected from the group consisting of ethanol, lactic acid, acetic acid, 1,3-propane diol, propanol, butanol, succinic acid, ethylene, glycerol, farnesol, geranylgeraniol and squalene.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a chart showing identities of amino acid sequences of XIs having activity in yeast with that of RsXI.

FIG. 2 is a chart showing sequence logo analysis results and motif analysis results of RsXI and other XIs having activity in yeast.

FIG. 3 is a chart showing amino acid sequence alignments of RsXI (SEQ ID NO: 1) and other XIs (SEQ ID NOS: 14 to 23) having activity in yeast.

FIG. 4 is a chart showing identities with respect to each motif of XIs having activity in yeast.

FIG. 5 is a graph showing growth test results (specific growth rates) utilizing xylose as a carbon source.

FIG. 6 is a graph showing fermentation test results (change of xylose and ethanol with time) utilizing xylose as a carbon source.

FIG. 7 is a chart showing fermentation test results (xylose consumption in 72 hours) utilizing xylose as a carbon source.

FIG. 8 is a chart showing fermentation test results (xylose consumption in 72 hours) utilizing xylose as a carbon source.

FIG. 9 is charts A to D showing fermentation test results (xylose consumption in 72 hours) with respect to various variant strains utilizing xylose as a carbon source.

DESCRIPTION OF EMBODIMENTS

The disclosure hereunder relates to a novel XI, which has certain relationship with RsXI, namely a xylose isomerase originated from an enteric protist in Reticulitermes speratus and is useful for enhancing xylose utilization ability of an eukaryotic cell such as yeast. The inventors have discovered that a substitution mutation effective in enhancing xylose utilization ability of yeast found for RsXI is also effective in enhancing xylose utilization ability of a eukaryotic cell with respect to another XI. Another XI having motifs common to RsXI is considered to be a XI having a function similar to RsXI. In the event that a modified XI, in which a substitution mutation is introduced to asparagine in a motif, is expressed in a eukaryotic cell, xylose isomerase activity can be exhibited and the xylose utilization ability of the host eukaryotic cell can be improved. The disclosure of the current description will be described below in detail referring appropriately to the drawings.

(Protein Having Xylose Isomerase Activity)

The present protein, when aligned with an amino acid sequence expressed by SEQ ID NO:1 of RsXI, may include 1, or 2 or more of the 1st to 6th motifs (SEQ ID NOs:2 to 7) described below. The 1st to 6th motifs may be contained in the present protein from the N-terminus side of the amino acid sequence in the order described.

All of the motifs are found in RsXI, and the inventors found the same also in other XIs by a motif analysis according to multiple alignments with such other XIs.

The present protein contains preferably at least the 6th motif out of the 1st to 6th motifs. The protein contains preferably also the 4th motif, more preferably also the 5th motif, still more preferably also the 3rd motif, even more preferably also the 1st motif, and still even more preferably also the 2nd motif.

In the motif analysis, the amino acid sequence of RsXI was searched by Protein BLAST (Database: Non-redundant protein sequence, Algorism parameter: default setting). With respect to other top 500 analogous amino acid sequences and the amino acid sequence of RsXI, alignment analysis was performed. Form the results of the alignment analysis, consensus sequences of 6 characteristic domains were defined as motif sequences.

Examples of such other XIs hit as analogous amino acid sequences include 10 XIs having activity in yeast shown in FIG. 1. The identities of the XIs with the amino acid sequence expressed by SEQ ID NO:1 of RsXI are 46% to 63% and not particularly high, however the XIs have in common the 1st to 6th motifs and have high identities with the respective motifs in SEQ ID NO:1. Such other XIs can be found in a publicly known database using the amino acid sequence of RsXI expressed by SEQ ID NO:1.

In FIG. 2 are shown the results of a sequence logo analysis and a motif analysis by multiple alignments of the amino acid sequence expressed by SEQ ID NO:1 and the amino acid sequences of the 10 XIs (SEQ ID NOs:14 to 23) shown in FIG. 1, as well as the identities with respect to each motif. The identities are described in FIG. 2 in descending order of identity percentage. FIG. 3 shows multiple alignment analysis results of RsXI and other 10 XIs. FIG. 4 shows identities with respect to each motif of other XIs having activity in yeast.

Those skilled in the art can perform multiple alignment by using various publicly known database such as Protein BLAST, which is an aforedescribed publicly known database. There is no particular restriction on a technique to be used for multiple alignment or a technique for obtaining a consensus sequence, and various techniques, such as ClustalW; HMMER (hidden Markov model); MultiAlin; and mkdom/xdom, can be applied. Further, from the multiple alignment highly conservative amino acids can be extracted. Such a technique is also well known to those skilled in the art. For example, using Weblogo3.3 a logo of highly conservative amino acids can be created. In the sequence logo analysis shown in FIG. 2, a higher conservation amino acid is represented larger. Further, from such a sequence logo analysis, a motif analysis to specify a high conservation region (motif) is possible.

“Identity” and “similarity” herein, as have been known well to those skilled in the art, are relationships between two or more proteins or two more polynucleotide determined by comparing the sequences. “Identity” in the art, also means the degree of sequence invariance between protein or polynucleotide sequences, as determined by the alignment between the protein or polynucleotide sequences, as the case maybe the alignment between strings of such sequences. In addition, “similarity” means the degree of sequence relatedness between protein or polynucleotide sequences, as determined by the alignment between the protein or polynucleotide sequences, as the case maybe the alignment between strings of such sequences. More specifically, “Similarity” is determined by the sequence identity or conservativeness (replacement which can maintain the physical and chemical properties of a particular amino acid or amino acid sequence). “Similarity” is referred to as Similarity in the search result BLAST sequence homology to be described later. Preferred methods of determining “identity” or “similarity” are designed to give the longest alignment between the sequences to be tested. Method for determining identity and similarity, are codified in publicly available computer programs. “Identity” and “similarity” can be determined by, for example, using the BLAST (Basic Local Alignment Search Tool) program by Altschul et. al., (for example, Altschul S F, Gish W, Miller W, Myers E W, Lipman D J, J. Mol Biol, 215: P 403-410 (1990), Altschyl S F, Madden T L, Schaffer A A, Zhang J, Miller W, Lipman D J, 25 Nucleic Acids Res. 25: p 3389-3402 (1997)). Where software such as BLAST used, it is but not limited to, preferable to use default values.

(The 1st Motif)

The 1st motif is expressed by FXXXXKXXXXXXXXHDXD (SEQ ID NO:2). The 1st motif is composed of 18 amino acids and corresponds to position 88 to position 105 of the amino acid sequence expressed by SEQ ID NO:1. It is presumed that in the motif the amino acid residues at position 15 (H) and position 18 (D) are residues constituting an active site (Hu, H., H. Liu, and Y. Shi., 1997. The reaction pathway of the isomerization of D-xylose catalyzed by the enzyme D-xylose isomerase: a theoretical study, Proteins 27: 545-55.).

The respective X's (naturally occurring amino acids) in the 1st motif expressed by SEQ ID NO:2 are preferably the following amino acids:

position 2: D or E

position 3: F or I or L or M

position 4: A or C or F or I or L or M or Y

position 5: D or E or H or N or Q or S or T

position 7: L or M

position 8: D or G or N or S

position 9: A or I or L or T or V

position 10: D or E or G or K or P

position 11: F or H or Y

position 12: F or L or W or Y

position 13: A or C or S or T

position 14: F or W, and

position 17: A or I or K or R or T or V.

The 1st motif is preferably expressed by FXXXXKXGXXXXXFHDXD (SEQ ID NO:8). Meanwhile, the 1st motif to the 6th motif expressed by SEQ ID NOs:8 to 13 were defined as domains that agree with the top 500 motif sequences obtained by the alignment analysis by performing another alignment analysis similar to the above, limitedly with the amino acid sequence of RsXI (SEQ ID NO:1) and 10 amino acid sequences (SEQ ID NO:14 to 23) of XIs, which activity in yeast was confirmed, shown in FIG. 1.

The respective X's (naturally occurring amino acids) in the 1st motif expressed by SEQ ID NO:8 are preferably the following amino acids:

position 2: D or E

position 3: F or I or L

position 4: A or M

position 5: E or Q or S or T

position 7: L or M

position 9: I or V

position 10: E or G or K or P

position 11: F or H or Y

position 12: F or W or Y

position 13: C or T, and

position 17: A or I or K or R or V.

The 1st motif is more preferably expressed by FEXXXKXGXXXXCFHDXD (SEQ ID NO:102). The respective X's (naturally occurring amino acids) in the 1st motif expressed by SEQ ID NO:102 are preferably the following amino acids. This 1st motif is based on the results of an alignment analysis performed with the amino acid sequence of RsXI (SEQ ID NO:1) and amino acid sequences of XIs originated from Piromyces sp. E2, Clostridium phytofermentans, Bacteroides thetaiotaomicron, and Lactococcus lactis respectively.

position 3: F or I or L

position 4: A or M

position 5: E or Q or S or T

position 7: L or M

position 9: I or V

position 10: E or K or P

position 11: F or Y

position 12: F or Y, and

position 17: A or I or V

The 1st motif is preferably composed of an amino acid sequence having an identity of 60% or more with the amino acid corresponding to the 1st motif of RsXI expressed by FEFMSKLGVEYFCFHDAD (SEQ ID NO:24). The 1st motif is more preferably composed of an amino acid sequence having an identity of 65% or more with the amino acid sequence expressed by SEQ ID NO:24, still more preferably 70% or more, and even more preferably 75% or more. The identity may be 80% or more, may be 85% or more, may be 90% or more, and further may be 95% or more.

As obvious from FIG. 2, with respect to the 1st motif, 10 XIs having activity in yeast shown in FIG. 1 have preferably an identity of 66% or more in terms of amino acid sequence identity, preferably 70% or more, more preferably 75% or more, and further preferably 80% or more.

(The 2nd Motif)

The 2nd motif is expressed by XXXXXXXWGGREGYXXLXNT (SEQ ID NO:3). The 2nd motif is composed of 20 amino acids and corresponds to position 182 to 201 of the amino acid sequence of RsXI expressed by SEQ ID NO:1.

The respective X's (naturally occurring amino acids) in the 2nd motif expressed by SEQ ID NO:2 are preferably the following amino acids:

position 1: D or G or K or N

position 2: A or G or S

position 3: A or E or K or Q or S or T or V

position 4: G or N

position 5: F or Y

6position: V or T

position 7: F or L

position 15: A or D or E or H or M

position 16: C or N or S or T, and

position 18: H or L or W.

The 2nd motif is preferably expressed by XXXXXVFWGGREGYXXLLNT (SEQ ID NO:9). The respective X's (naturally occurring amino acids) in the 2nd motif expressed by SEQ ID NO:9 are preferably the following amino acids:

position 1: G or N

position 2: A or G

position 3: V or K or E or T

position 4: G or N

position 5: F or Y

position 15: E or M or H, and

position 16: S or T.

The 2nd motif is more preferably expressed by GXXXYVFWGGREGYXXLLNT (SEQ ID NO:103). The respective X's (naturally occurring amino acids) in the 2nd motif expressed by SEQ ID NO:103 are preferably the following amino acids. This 2nd motif is based on the results of an alignment analysis performed with the amino acid sequence of RsXI (SEQ ID NO:1) and amino acid sequences of XIs originated from Piromyces sp. E2, Clostridium phytofermentans, Bacteroides thetaiotaomicron, and Lactococcus lactis respectively.

position 2: A or G

position 3: V or K or E

position 4: G or N

position 15: E or M, and

position 16: S or T

The 2nd motif is preferably composed of an amino acid sequence having an identity of 60% or more with the amino acid corresponding to the 2nd motif of RsXI expressed by GGVGYVFWGGREGYETLLNT (SEQ ID NO:25). The 2nd motif is more preferably composed of an amino acid sequence having an identity of 65% or more with the amino acid sequence expressed by SEQ ID NO:25, still more preferably 70% or more, and even more preferably 75% or more. The identity may be 80% or more, may be 85% or more, may be 90% or more, and further may be 95% or more.

As obvious from FIG. 2, with respect to the 2nd motif, 10 XIs having activity in yeast shown in FIG. 1 have preferably an identity of amino acid sequence of 75% or more, preferably 80% or more, more preferably 85% or more, still more preferably 90% or more, and even more preferably 95% or more.

(The 3rd Motif)

The 3rd motif is expressed by XXXXXXXXEPKPXEPXXHQYDXD (SEQ ID NO:4). The 3rd motif is composed of 23 amino acids and corresponds to position 225 to position 247 of the amino acid sequence of RsXI expressed by SEQ ID NO:1. It is presumed that in the motif the amino acid residues at position 9 (E) and position 11 (K) are residues constituting an active site.

The respective X's (naturally occurring amino acids) in the 3rd motif expressed by SEQ ID NO:4 are preferably the following amino acids:

position 1: G or N

position 2: F or H or Y

position 3: D or E or K or L or N or Q or R or T

position 4: G or P

position 5: A or D or I or N or Q or T

position 6: F or L or M

position 7: F or L or Y

position 8: I or L

position 13: K or M or Q

position 16: M or S or T

position 17: K or S or T, and

position 22: F or T or V or Y.

The 3rd motif is preferably expressed by XXXXXFXIEPKPXEPXXHQYDXD (SEQ ID NO:10). The respective X's (naturally occurring amino acids) in the 3rd motif expressed by SEQ ID NO:10 are preferably the following amino acids:

position 1: G or N

position 2: F or H

position 3: K or D or T or E or L

position 4: G or P

position 5: D or Q or T or I

position 7: F or L or Y

position 13: K or M

position 16: M or S or T

position 17: K or T, and

position 22: F or V or Y.

With respect to the 3rd motif in the amino acid sequence expressed by SEQ ID NO:10, based on the results of an alignment analysis performed with the amino acid sequence of RsXI (SEQ ID NO:1) and amino acid sequences of XIs originated from Piromyces sp. E2, Clostridium phytofermentans, Bacteroides thetaiotaomicron, and Lactococcus lactis respectively, the following amino acids are preferable:

position 1: G or N

position 2: F or H

position 3: K or D or L

position 4: G or P

position 5: D or T or I

position 7: L or Y

position 13: K or M

position 16: M or T

position 17: K or T, and

position 22: F or V.

The 3rd motif is preferably composed of an amino acid sequence having an identity of 60% or more with the amino acid corresponding to the 3rd motif of RsXI expressed by GFKGDFYIEPKPKEPTKHQYDFD (SEQ ID NO:26). The 3rd motif is more preferably composed of an amino acid sequence having an identity of 65% or more with the amino acid sequence expressed by SEQ ID NO:26, still more preferably 70% or more, and even more preferably 75% or more. The identity may be 80% or more, may be 85% or more, may be 90% or more, and further may be 95% or more.

As obvious from FIG. 2, with respect to the 3rd motif, 10 XIs having activity in yeast shown in FIG. 1 have preferably an identity of amino acid sequence of 65% or more, preferably 70% or more, more preferably 75% or more, still more preferably 80% or more, even more preferably 85% or more, still even more preferably 90% or more, and yet even more preferably 95% or more.

(The 4th Motif)

The 4th motif is expressed by LXXXXXXNXEXNHXXLXXHXXXH (SEQ ID NO:5). The 4th motif is composed of 23 amino acids and corresponds to position 260 to position 282 of the amino acid sequence of RsXI expressed by SEQ ID NO:1. It is presumed that in the motif the amino acid residues at position 10 (E) and position 13 (K) are residues constituting an active site.

The respective X's (naturally occurring amino acids) in the 4th motif expressed by SEQ ID NO:5 are preferably the following amino acids:

position 2: D or E or K or L or N or Q

position 3: D or E or G or K or N or P or Q

position 4: D or E or H or Y

position 5: F or I or V

position 6: K or R

position 7: F or I or L or M or V

position 9: I or L

position 11: A or G or P or T or V

position 14: A or T

position 15: N or T or W

position 17: A or S

position 18: F or G or Q

position 20: C or D or S or T

position 21: F or H or M or Y, and

position 22: D or E or H or M or Q.

The 4th motif is preferably expressed by LXXXFKXNXEXNHXXLAGHXXXH (SEQ ID NO:11). The respective X's (naturally occurring amino acids) in the 4th motif expressed by SEQ ID NO:11 are preferably the following amino acids:

position 2: D or E or N

position 3: K or Q

position 4: D or Y

position 7: I or L or M or V

position 9: I or L

position 11: A or P or T or V

position 14: A or T

position 15: T or W

position 20: C or T

position 21: F or H, and

position 22: Q or E.

The 4th motif is more preferably expressed by LXKXFKXNXEXNHAXLAGHTFXH (SEQ ID NO:104). The respective X's (naturally occurring amino acids) in the 4th motif expressed by SEQ ID NO:104 are preferably the following amino acids. This 4th motif is based on the results of an alignment analysis performed with the amino acid sequence of RsXI (SEQ ID NO:1) and amino acid sequences of XIs originated from Piromyces sp. E2, Clostridium phytofermentans, Bacteroides thetaiotaomicron, and Lactococcus lactis respectively.

position 2: D or E

position 4: D or Y

position 7: L or M or V

position 9: I or L

position 11: A or T or V

position 15: T or W, and

position 22: Q or E

The 4th motif is preferably composed of an amino acid sequence having an identity of 60% or more with the amino acid corresponding to the 4th motif of RsXI expressed by LEKDFKLNIEANHATLAGHTFQH (SEQ ID NO:27). The 4th motif is more preferably composed of an amino acid sequence having an identity of 65% or more with the amino acid sequence expressed by SEQ ID NO:27, still more preferably 70% or more, and even more preferably 75% or more. The identity may be 80% or more, may be 85% or more, may be 90% or more, and further may be 95% or more.

As obvious from FIG. 2, with respect to the 4th motif, 10 XIs having activity in yeast shown in FIG. 1 have preferably an identity of amino acid sequence of 73% or more, preferably 75% or more, more preferably 80% or more, still more preferably 85% or more, and even more preferably 90% or more.

(The 5th Motif)

The 5th motif is expressed by XGSXDXNXGXXXXGWDXDXXP (SEQ ID NO:6). The 5th motif is composed of 21 amino acids and corresponds to position 293 to position 303 of the amino acid sequence of RsXI expressed by SEQ ID NO:1. It is presumed that in the motif the amino acid residues at position 5 (D), position 16 (D) and position 18 (D) are residues constituting an active site.

The respective X's (naturally occurring amino acids) in the 5th motif expressed by SEQ ID NO:6 are preferably the following amino acids:

position 1: F or L

position 4: I or L or V

position 6: A or S

position 8: M or Q or R or T

position 10: D or H or N or S

position 11: A or K or L or M or P or T or V or Y

position 12: L or N or Q or D

position 13: I or L or N

position 17: I or T

position 19: E or Q, and

position 20: F or Y.

The 5th motif is preferably expressed by XGSXDXNXGXXXXGWDTDXFP (SEQ ID NO:12). The respective X's (naturally occurring amino acids) in the 5th motif expressed by SEQ ID NO:12 are preferably the following amino acids:

position 1: F or L

position 4: I or V

position 6: A or S

position 8: Q or R or T

position 10: D or N or S

position 11: L or M or P or Y

position 12: D or L or N or Q

position 13: L or N, and

position 19: E or Q.

The 5th motif is more preferably expressed by XGSXDANXGXXXXGWDTDXFP (SEQ ID NO:105). The respective X's (naturally occurring amino acids) in the 5th motif expressed by SEQ ID NO:105 are preferably the following amino acids. This 5th motif is based on the results of an alignment analysis performed with the amino acid sequence of RsXI (SEQ ID NO:1) and amino acid sequences of XIs originated from Piromyces sp. E2, Clostridium phytofermentans, Bacteroides thetaiotaomicron, and Lactococcus lactis respectively.

position 1: F or L

position 4: I or V

position 8: Q or R or T

position 10: D or N

position 11: P or Y

position 12: L or N or Q

position 13: L or N, and

position 19: E or Q

The 6th motif is preferably composed of an amino acid sequence having an identity of 60% or more with the amino acid corresponding to the 5th motif of RsXI expressed by LGSVDANTGDPLLGWDTDEFP (SEQ ID NO:28). The 5th motif is more preferably composed of an amino acid sequence having an identity of 65% or more with the amino acid sequence expressed by SEQ ID NO:28, still more preferably 70% or more, and even more preferably 75% or more. The identity may be 80% or more, may be 85% or more, may be 90% or more, and further may be 95% or more.

As obvious from FIG. 2, with respect to the 5th motif, 10 XIs having activity in yeast shown in FIG. 1 have preferably an identity of amino acid sequence of 71% or more, preferably 75% or more, and more preferably 80% or more.

(The 6th motif)

The 6th motif is expressed by GGXNFDXKXRR (SEQ ID NO:7). The 6th motif is composed of 11 amino acids and corresponds to position 335 to position 345 of the amino acid sequence of RsXI expressed by SEQ ID NO:1. It is presumed that in the motif the amino acid residue at position 6 (D) is a residue constituting an active site.

The respective X's (naturally occurring amino acids) in the 6th motif expressed by SEQ ID NO:7 are preferably the following amino acids:

position 3: F or I or L or M or T or V

position 7: A or C or S or T, and

position 9: I or L or P or T or V.

The 6th motif is preferably expressed by GGXNFDXKXRR (SEQ ID NO:13). The respective X's (naturally occurring amino acids) in the 6th motif expressed by SEQ ID NO:13 are preferably the following amino acids:

position 3: F or I or L or T

position 7: A or C or S, and

position 9: T or V.

The respective X's (naturally occurring amino acids) in the 6th motif expressed by SEQ ID NO:13 are preferably the following amino acids. This 6th motif is based on the results of an alignment analysis performed with the amino acid sequence of RsXI (SEQ ID NO:1) and amino acid sequences of XIs originated from Piromyces sp. E2, Clostridium phytofermentans, Bacteroides thetaiotaomicron, and Lactococcus lactis respectively.

position 3: I or L or T

position 7: A or S, and

position 9: T or V

The 6th motif is preferably composed of an amino acid sequence having an identity of 60% or more with the amino acid corresponding to the 6th motif of RsXI expressed by GGLNFDSKVRR (SEQ ID NO:29). The 6th motif is more preferably composed of an amino acid sequence having an identity of 65% or more with the amino acid sequence expressed by SEQ ID NO:29, still more preferably 70% or more, and even more preferably 75% or more. The identity may be 80% or more, may be 85% or more, may be 90% or more, and further may be 95% or more.

As obvious from FIG. 2, with respect to the 6th motif, 10 XIs having activity in yeast shown in FIG. 1 have preferably an identity of amino acid sequence of 72% or more, preferably 75% or more, more preferably 80% or more, still more preferably 85% or more, and even more preferably 90% or more.

The present protein has an amino acid sequence, in which asparagine (N) in the 6th motif is another amino acid. In other words, it has an amino acid sequence, in which asparagine in the 6th motif is substituted with another amino acid. The asparagine in the 6th motif is considered to have important value for improving the xylose utilization (fermentation) ability of a eukaryotic cell owing to a XI having certain relationship with RsXI.

There is no particular restriction on a substituting other amino acid for N in the 6th motif, and those skilled in the art can specify the same by introducing a point mutation to the position of asparagine in the 6th motif by a publicly known mutagenesis method to generate a modified protein, introducing the modified protein into a eukaryotic cell such as yeast, and comparing the improved xylose utilization ability with a wild type protein.

Examples of a preferable other amino acid include cysteine, threonine, valine and alanine. A mutation by substituting any of the above amino acids for asparagine is preferable. In some cases, cysteine or threonine is more preferable.

All of the identities with respect to the 1st to the 6th motifs of the present protein are preferably 65% or more, and more preferably any of the identities are 70% or more.

Further, all of the identities with respect to the 2nd to the 4th motifs of the present protein are preferably 75% or more, and more preferably 80% or more.

Further, the present protein has preferably an amino acid sequence having an identity of 45% or more with respect to an amino acid sequence of RsXI expressed by SEQ ID NO:1 and more preferably an amino acid sequence having an identity of 50% or more.

Amino acid sequences within a predetermined range of identity with respect to the respective amino acid sequences of the various motifs and the amino acid sequence expressed by SEQ ID NO:1 of RsXI are amino acid sequences derived by deletion, substitution, or addition of one or several amino acids from the amino acid sequence in question. A mutation of an amino acid in an amino acid sequence, namely deletion, substitution, and addition, may occur singly or in combination of 2 or more types thereof. Further, there is no particular restriction on the total number of mutations, insofar as the identity is within a specified range.

The present protein has xylose isomerase activity. “XI activity” is the activity of isomerizing xylose into xylulose. XI activity can be measured by known methods using the reduction in the amount of xylose as the substrate of this isomerization reaction, or the amount of xylulose produced by the reaction. “Having XI activity” simply means that there is XI activity. Preferably, this means that the XI activity is equivalent to or greater than that of a protein consisting of an amino acid sequence represented by SEQ ID NO: 1 or any one of SEQ ID NO: 14-23. The XI activity can be measured based on the consumption amount or the consumption rate of xylose or production amount of xylulose by the present protein or XI content fraction such as cell lysate of the present protein expressing eukaryotic cell such as yeast. The XI activity is preferably at least 70% or more preferably at least 80% or still more preferably at least 90% or most preferably at least 100% of the XI activity of the protein consisting of an amino acid sequence represented by SEQ ID NO: 1 or any one of SEQ ID NO: 14-23 or the present protein which has Asparagine at the specified site (typically wild type xylose isomerase).

When the present protein is expressed in a eukaryotic cell such as yeast, the xylose utilization ability of the eukaryotic cell is preferably higher than the xylose utilization ability evaluated under the same conditions for a protein equivalent to the present protein, in which the specific position is asparagine (typically, a protein having wild type xylose isomerase activity (wild type protein)). Xylose utilization ability is evaluated, for example, by the growth amount (rate) of a eukaryotic cell, the xylose consumption amount (rate), the fermentation production amount (e.g. ethanol), or the like in the presence of xylose. This is because xylose utilization ability is the ultimately required function. The xylose utilization ability is preferably 110% or more as high as that of the wild type protein, more preferably 120% or more, still more preferably 130% or more, even more preferably 150% or more, still even more preferably 200% or more, yet even more preferably 250% or more, and most preferably 300% or more.

Examples of the present protein include proteins having amino acid sequences containing an amino acid other than asparagine at the asparagine position of the 6th motif in SEQ ID NO:1 and SEQ ID NOs:14 to 23. Namely, with respect to RsXI, examples include a protein having the amino acid sequence expressed by SEQ ID NO:30, with respect to a XI originated from Clostridium phytofermentans a protein having the amino acid sequence expressed by SEQ ID NO:31, with respect to a XI originated from Clostridium difficile a protein having the amino acid sequence expressed by SEQ ID NO:32, with respect to a XI originated from Fusobacterium mortiferum a protein having the amino acid sequence expressed by SEQ ID NO:33, with respect to a XI originated from Bacteroides thetaiotaomicron a protein having the amino acid sequence expressed by SEQ ID NO:34, with respect to a XI originated from Cyllamyces aberensisn a protein having the amino acid sequence expressed by SEQ ID NO:35, with respect to a XI originated from Bacteroides fragilis a protein having the amino acid sequence expressed by SEQ ID NO:36, with respect to a XI originated from Orpinomyces sp. ukk1 a protein having the amino acid sequence expressed by SEQ ID NO:37, with respect to a XI originated from Piromyces sp. E2 a protein having the amino acid sequence expressed by SEQ ID NO:38, with respect to a XI originated from Lactococcus lactis a protein having the amino acid sequence expressed by SEQ ID NO:39, and with respect to a XI originated from Ciona intestinals a protein having the amino acid sequence expressed by SEQ ID NO:40.

Further, the present protein includes, for example, proteins having an amino acid sequence having an identity of 70% or more, preferably 75% or more, still more preferably 80% or more, even more preferably 85% or more, still even more preferably 90% or more, yet even more preferably 95% or more, and most preferably 98% or more with any of amino acid sequences expressed by SEQ ID NO:1, and SEQ ID NOs:14 to 23, as well as an amino acid other than asparagine (preferable examples are cysteine, threonine, valine, and alanine; a mutation substituting any of the amino acids for asparagine is preferable; and in some cases cysteine and threonine are preferable) substituting for asparagine at a position corresponding to position 337 of SEQ ID NO:1. Meanwhile, the expression “position corresponding to” means, when alignment of an amino acid sequence to be compared having a certain amino acid sequence identity with respect to a base amino acid sequence such as SEQ ID NO:1 is performed, a position of the amino acid sequence to be compared corresponding to a specific position of the base amino acid sequence. In amino acid sequences expressed by SEQ ID NOs:14 to 23, positions corresponding to the position 337 are position 337, position 337, position 335, position 339, position 338, position 339, position 338, position 338, position 337, and position 357.

The present protein is preferably protein having an amino acid sequence having an identity of 70% or more, preferably 75% or more, more preferably 75% or more, still more preferably 80% or more, even more preferably 85% or more, still even more preferably 90% or more, yet even more preferably 95% or more, and most preferably 98% or more with any of amino acid sequences expressed by SEQ ID NO:1, SEQ ID NOs:14, 17, 21 and 22, and having asparagine at a position corresponding to position 337 of SEQ ID NO:1 substituted with an amino acid other than asparagine (preferable examples are cysteine, threonine, valine, and alanine; a mutation substituting any of the amino acids for asparagine is preferable; and in some cases cysteine and threonine are preferable).

The present protein is available by various methods. For example, the present protein can be obtained by extracting a protein having the 1st to 5th motifs at identities not less than a certain level by means of a publicly known homology search, motif analysis, or the like using an amino acid sequence selected from the group consisting of SEQ ID NOs:1 and 14 to 23, or a nucleotide sequence coding for the amino acid sequence as a query sequence; and introducing a mutation to an asparagine position in the 5th motif of the extracted protein. Site-specific introduction of a mutation into an amino acid sequence is possible by those skilled in the art according to a publicly known technique. Example of a method for preparing DNA coding for a protein with a modified amino acid sequence well known to those skilled in the art include a site-directed mutagenesis method (Kramer W, and Fritz H-J: Methods Enzymol 154: 350, 1987).

Alternatively, proteins containing an amino acid other than asparagine at a specific asparagine may be extracted using, for example, SEQ ID NOs:30 to 40 substituting a specific asparagine position with another amino acid in SEQ ID NOs:1 and 14 to 23, or a nucleotide sequence coding for the same as a query sequence. Also in the natural world, by a mutation of a nucleotide sequence, a mutation of the encoded amino acid sequence of a protein may take place.

Further, a DNA may be isolated by a hybridization technique using a DNA coding for an amino acid sequence expressed by SEQ ID NOs:1, 14 to 23, or 30 to 40, or the complementary strand thereof as a probe, and a wild type of the present protein encoded by the DNA may be obtained, followed by modification; or the present protein may be obtained directly. Further, using an oligonucleotide which hybridizes specifically with the DNA or the complementary strand as a primer, a wild type of the present protein may be obtained by a PCR reaction, followed by modification, or the present protein may be obtained directly. Acquisition of the present protein as above can be performed routinely by those skilled in the art.

With respect to a hybridization technique, a hybridization reaction should preferably be carried out under a stringent condition. The stringent condition will be described below.

The present protein is prepared by transforming an appropriate host, such as a eukaryotic cell, by a DNA construct containing a DNA coding for the present protein, culturing the transformed host cell by an ordinary method well known to those skilled in the art, and harvesting the present protein from the cultured cells or culture medium. The technique is well known to those skilled in the art.

(DNA Coding for the Present Protein)

The present DNA is a DNA with a nucleotide sequence coding for the present protein. The present DNA can be obtained by preparing synthetically a DNA coding for the present protein, or as described above, by a site-directed mutagenesis method, a hybridization technique, a PCR, or the like.

Stringent condition in hybridization refers to conditions, for example in which so-called specific hybrid is formed, a non-specific hybrid is not formed. For example, a condition such that complementary strand of the DNA having high identity such as at least 70% identical, preferably at least 80% identity, more preferably at least 85%, or still more preferably at least 90%, or most preferably at least 95% identity with the nucleotide sequence represented by any one of SEQ ID NOS: 1, 3, 5, 7, 9, 11 or 13 hybridizes with the DNA while the complementary strand of the DNA having the lower identity does not hybridize with the DNA is included. Typically, Na salt concentration is 15 to 750 mM, preferably 50 to 750 mM, more preferably 300 to 750 mM, temperature is 25 to 70 deg C., preferably 50 to 70 deg C., more preferably 55 to 65, and formamide concentration is 0 to 50%, preferably 20 to 50%, more preferably 35 to 45%. Further, stringent condition includes filter washing condition after hybridization which Na salt concentration is 15 to 600 mM, preferably 50 to 600 mM, more preferably 300 to 600 mM and temperature is 50 to 70 deg C., preferably 55 to 70 deg C., more preferably 60 to 65 deg C., typically.

In a nucleotide sequence coding for a specific amino acid sequence, at least one base in a nucleotide sequence coding for the predetermined amino acid sequence can be substituted with another kind of base as per degeneracy in genetic coding without changing an amino acid sequence of a protein. Therefore, the present DNA includes a DNA coding for a nucleotide sequence modified by substitution as per degeneracy in genetic coding. The present DNA may be constituted with a nucleotide sequence optimized for expression in a eukaryotic cell, such as yeast.

(Vector for Transformation)

The vector for transformation disclosed herein retains the present DNA downstream of an appropriate promoter as operable by the promoter. Examples of the promoter include various promoters functioning in a eukaryotic cell, etc. as described below, and inductive promoters, such as a GAL promoter. The recombinant vector for transformation may be further provided with a terminater, an enhancer, a replication origin (ori), a marker, etc., and such elements may be selected appropriately according to need. Further, in the event that the recombinant vector is intended to implant a desired DNA fragment into a chromosome, as for gene substitution, the same has a homologous domain corresponding to a predetermined domain on the chromosome. Further, the present vector can be constructed utilizing an appropriately selected commercially available yeast expression vector, etc.

Such general manipulations required for constructing a recombinant vector are conducted routinely by those skilled in the art, and those skilled in the art can carry out the same by referring appropriately to a handbook of experimental techniques, for example, T. Maniatis, J. Sambrook, et al. “Molecular Cloning, A Laboratory Manual”, Cold Spring Harbor Laboratory, 1982, 1989, 2001.

(Eukaryotic Cell)

The eukaryotic cell disclosed herein is a eukaryotic cell containing the present DNA. The present eukaryotic cell is typically a transformed eukaryotic cell transformed by the present vector. The DNA may be retained outside a chromosome in a host cell, but is preferably retained on a chromosome. Further, for exhibiting high xylose utilization ability, it is preferable, for example, that a plurality of copies thereof are retained.

There is no particular restriction on a eukaryotic cell as a host of a transformant disclosed hereunder. From the standpoint of substance production and the like, it may be an Aspergillus or other mold or yeast. Examples of Aspergillus species include Aspergillus aculeatus, Aspergillus orizae and the like. Examples of yeasts include various known yeasts including Saccharomyces cerevisiae and other Saccharomyces yeasts, Schizosaccharomyces pombe and other Schizosaccharomyces yeasts, Candida shehatae and other Candida yeasts, Pichia stipitis and other Pichia yeasts, Hansenula yeasts, Klocckera yeasts, Schwanniomyces yeasts and Yarrowia yeasts, Trichosporon yeasts, Brettanomyces yeasts, Pachysolen yeasts, Yamadazyma yeasts, Kluyveromyces marxianus, Kluyveromyces lactis and other Kluyveromyces yeasts, Issatchenkia orientalis and other Issatchenkia yeasts and the like. Of these, a Saccharomyces yeast is preferred from the standpoint of industrial utility and the like. Of these, Saccharomyces cerevisiae is preferred.

The DNA is carried by the host in such a way that it can be expressed. That is, it may be linked under the control of a suitable promoter, and a terminator, enhancer, replication origin (ori), marker or the like may also be provided. The promoter may be inductive or constitutive. Examples of constitutive promoters in yeasts include the 3-phosphoglycerate kinase (PGK) promoter, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) promoter, alcohol dehydrogenase 1 (ADH1) promoter, histidine nutritional function gene (HIS3) promoter, cytochrome bc1 complex (CYC1) promoter and hyperosmolarity responsive 7 gene (HOR7) promoter and modifications of these.

The eukaryotic cell may also be one that secretorily expresses a cellulase or hemicellulase either extracellularly or on the cell surface. Examples include endoglucanase, cellobiohydrolase, β-glucosidase and various other cellulases as well as hemicellulase and other biomass degrading enzymes. Expression of such proteins allows for effective utilization of sugars other than lignin derived from lignocellulose. The transformant disclosed in this Description may also be one that has been given genetic engineering modifications as necessary, such as introduction of an exogenous gene or disruption of an endogenous gene.

The eukaryotic cell may be one capable of producing desired useful chemicals by fermentation as explained below. An eukaryotic cell capable of producing a useful chemical may be provided with an endogenous gene and/or exogenous gene involved in producing the useful chemical. A desired endogenous gene may also be disrupted. Yeasts ordinarily produce ethanol by anaerobic fermentation, but a host that has been transformed by genetic engineering modifications or the like to make it capable of producing another useful chemical is also possible. Examples of useful chemicals include not only ethanol but also lactic acid, acetic acid, 1,3-propane-diole, propanol, butanol, succinic acid, ethylene and glycerol. Preferably the transformant is capable of producing one or two or more of these as useful substances. The host of the transformant disclosed in this Description may comprise a genetic modification or the like to yeast or the like that produces an organic acid such as lactic acid (Japanese Patent Application Publication No. 2003-259878, Japanese Patent Application Publication No. 2006-006271, Japanese Patent Application Publication No. 2006-20602, Japanese Patent Application Publication No. 2006-75133, Japanese Patent Application Publication No. 2006-2966377 and Japanese Patent Application Publication No. 2007-89466).

Introducing methods of the vector into the host cell include calcium phosphate method, transformation, transfection, conjugation, protoplast fusion, electroporation, lipofection, lithium acetate method and any other methods known to the art. These techniques are described in published books including the above mentioned text. The transformant of the present Description can be obtained by screening by the marker gene or the expression of the activity of the gene among yeast which the vector is introduced.

(Method of Producing Useful Chemical)

The useful chemical production method disclosed in this Description is provided with a step of culturing the eukaryotic cell in the presence of xylose. Because the eukaryotic cell has xylose utilization ability, it can effectively use any xylose contained as a carbon source, and convert it into a useful substance in the production method disclosed in this Description. Thus, even when the medium contains saccharides of lignocellulose including xylose, this biomass carbon source can be effectively utilized and converted into a useful substance. In addition to xylose, the lignocellulose saccharides may include glucose, as well as hemicellulose decomposition products.

Xylose includes arabinoxylan, glucuronoxylan and other xylans. In nature, these polymers form one component of hemicellulose, and are present in lignocellulose and other forms of biomass and the like. Xylose can be obtained by digesting xylans with an endoxylanase, xylosidase or the like.

The useful chemical may also be a compound that is not an intrinsic metabolite, but one that the yeast has been made capable of synthesizing by a genetically engineered substitution, addition or the like in one or two or more enzymes in the glucose metabolism system. Examples of useful chemicals include ethanol as well as lower alcohols, lactic acid, acetic acid and other organic acids. In addition, 1,3-propane-diol, propanol, butanol, succinic acid, glycerol and ethylene, farnesol, geranylgeraniol, squalene and other terpenoids and fine chemicals (coenzyme Q10, vitamins and other raw materials and the like) obtained by addition of isoprenoid synthesis pathways. Further, glycerin, plastics, synthetic raw materials and the like obtained by modifications in the glycolytic system and other materials used in biorefinery technology are included. As yeast has high performance of alcohol fermentation, the transformant can produce ethanol effectively in the medium with carbon source including xylose. An yeast having high performance of alcohol fermentation has high performance of an organic acid and other useful substances by modifications in the glycolytic system.

In the step of culturing, a medium which contains xylose as a carbon source is used. Further, the medium can contain glucose. Preferably, the carbon sources which are derived from biomass carbon source including lignocellulose. In addition, when yeast expresses cellulases and has an ability to metabolize cellulose, cellulose or the partial degradation products thereof can be included in the medium.

The culturing step can be accomplished according to a culture condition selected appropriately from the general culture conditions applied to the host cell of the eukaryotic cell. Typically, static culture, shaking culture or aerated stirred culture or the like can be used as the culture for fermentation. The aeration conditions can be set appropriately as anaerobic conditions, microaerobic conditions or aerobic conditions. The culture temperature is not particularly limited, and can be in the range of 25 deg C. to 55 deg C. The culture time can be set as necessary, and can be a few hours to about 150 hours. The pH can be adjusted with an inorganic or organic acid or alkali solution or the like. An antibiotic such as ampicillin or tetracycline can be added to the medium as necessary during culture.

By means of the culturing step, a useful chemical is produced according to the useful substance production ability of the microorganism used. For example, ethanol is obtained with the eukaryotic cell that has the ability to produce ethanol. The eukaryotic cell that has the ability to produce lactic acid and other organic acids due to biogenetic modification or the like can be used to produce lactic acid and the like. After completion of the useful substance production step, there can be a step in which the fraction containing the useful substance is collected from the culture liquid, and another step in which it is purified or concentrated. The processes for collection, purification and other process can be selected appropriately according to the type of useful substance and the like.

The useful substance production step may be followed by a step of collecting a useful substance-containing fraction from the culture liquid, and a further step of refining or concentrating this fraction. The collection step and refining or other step can be selected appropriately according to the type of useful substance and the like.

(Screening Method of Protein Having Xylose Isomerase Activity)

The present specification provides a screening method of protein having xylose isomerase activity. The present screening method can comprise a step of assessing xylose isomerase activity of modified protein when aligned with an amino acid sequence expressed by SEQ ID NO: 1, where the modified protein contains the following 1st to 6th motifs from the N-terminus of the protein in the order described, and has, in place of asparagine (N) in an amino acid sequence of the 6th motif of the protein having xylose isomerase activity, another amino acid. According to the method, the modified protein improved with respect to xylose isomerase activity can be obtained. Especially, a modified protein that is useful for expression in yeast can be obtained. The aforementioned another amino acid to be substituted is selected from various naturally occurring amino acids, and among these, cysteine, threonine, alanine and valine, in particular, cysteine and threonine can be exemplified. As for the 1^(st) to 6^(th) motifs in the modified protein, it is possible to apply the preferred embodiments described above.

As the protein source for obtaining such a modified protein, the amino acid sequences of RsXI can be searched as a query sequence by Protein BLAST (Database used the Non-redundant protein sequence, and Algorism parameter was in a default setting), and the top 500 species of the other similar amino acid sequences can be used; and among them, preferably the top 400 species, the top 300 species, the top 200 species, or the top 100 species of proteins may be exemplified. The position of asparagine in the 6^(th) motif may be identified using alignment analysis as described above. Notably, the modified protein to be subjected to screening can be obtained by a method of obtaining proteins as discussed above.

(Process for Producing a Protein Having Xylose Isomerase Activity)

According to this specification, a process for producing a protein having xylose isomerase activity is also provided. The present production method can comprise a step of producing a protein having xylose isomerase activity, which is a modified protein that, when aligned with an amino acid sequence expressed by SEQ ID NO: 1, contains the following 1st to 6th motifs from the N-terminus of the protein in the order described, and has, in place of asparagine (N) in an amino acid sequence of the 6th motif of the protein having xylose isomerase activity, another amino acid. According to the method, the modified protein having xylose isomerase activity can easily be produced. For the various aspects of other amino acids, protein source, and motifs, various aspects described hereinabove can be adapted similar to the screening method described above.

EMBODIMENTS

The present teaching is explained in detail below using examples, but the present invention is not limited by these examples. The genetic recombination operations described below were performed in accordance with Molecular Cloning: A Laboratory Manual (T. Maniatis, et al., Cold Spring Harbor Laboratory)

The compositions of culture media used in the following Examples are as follows:

SD agar culture medium: 6.7 g/L Yeast Nitorogen Base without amino acid, 20 g/L D-Glucose, and 20 g/L Agar

SX liquid culture medium: 6.7 g/L Yeast Nitorogen Base without amino acid, and 20 g/L D-Xylose

SX agar culture medium: 6.7 g/L Yeast Nitorogen Base without amino acid, 20 g/L D-Xylose, and 20 g/L Agar

SX liquid culture medium 50: 6.7 g/L Yeast Nitorogen Base without amino acid, and 50 g/L D-Xylose

First Embodiment

(Introduction of Mutation in RsXI Gene by Error-Prone PCR)

An error-prone PCR was carried out using GeneMorphII (by Stratagene) with pRS436GAP-RsXIC1-O, to which a xylose isomerase gene RsXI-C1-opt (GenBank: HV438143) originated from an enteric protist of Reticulitermes speratus optimized to a yeast codon was inserted, as a template. The reaction was carried out for 30 cycles with a cycle of at 95 deg C. for 2 min, 95 deg C. for 1 min, 60 deg C. for 1 min, and 72 deg C. for 1 min 30 sec, and followed by a reaction at 72 deg C. for 10 min. The sequences of the used primers were as follows:

pRSSacII-AAA-ATG-F4: (SEQ ID NO: 41) 5′-GAACTTAGTTTCGAATAAACACACATAAACAAACAAACCGCGG AAAATG-3′, and pRSXhoI-TAA-R3: (SEQ ID NO: 42) 5′-GTGAATGTAAGCGTGACATAACTAATTACATGATGCGGCCCTC GAGTTA-3′.

An amplified DNA fragment was cloned to PCR-Blunt II TOPO using a Zero Blunt TOPO PCR cloning kit (by Invitrogen) and the inserted DNA fragment was analyzed. As the result, it was confirmed that average 3 mutations per 1000 bases in the DNA fragment (error rate 0.3%) were introduced randomly.

Second Embodiment

(Construction of Yeast Gene Expression-Basic Plasmid)

A low-copy type transgenic vector pRS316GAP was constructed. A PCR was conducted with pRS436GAP (DDBJ accession number: AB304862) as a template using primers TDH3p-CYC1t-IF-F and R. The PCR was carried out using a PrimeSTAR HS DNA polymerase (Takara Bio Inc.) with a cycle of at 98 deg C. for 10 sec, 55 deg C. for 15 sec, 72 deg C. for 1 min 30 sec and by repeating 30 cycles. The sequences of the used primers were as follows:

TDH3p-CYC1t-IF-F: (SEQ ID NO: 43) 5′-TCGCTATTACGCCAGCTGGCGAAAGGGGGATGTGC-3′, and TDH3p-CYC1t-IF-R: (SEQ ID NO: 44) 5′-GATTCATTAATGCAGCTGGCACGACAGGTTTCCCGACTG GAAAGC-3′.

The produced DNA fragment was inserted using an In-Fusion Advantage PCR cloning kit (Takara Bio Inc.) in pRS316 (NBRP accession number: BYP562) digested by a restriction enzyme PvuII. The obtained plasmid was designated as pRS316GAP.

Third Embodiment

(Introduction of DNA Originated from Variant XI Gene Library into Yeast)

A DNA fragment produced by an error-prone PCR or a DNA fragment (control) produced by a PCR using pRS436-GAP-RsXIC1-O as a template, pRSSacII-AAA-ATG-F4 and pRsXhoI-TAA-R3 as primers, and a PrimeSTAR HS DNA polymerase as a polymerase were mixed with pRS316GAP digested by restriction enzymes SacII and XhoI, and introduced in yeast of Saccharomyces cerevisiae W600W strain (see Japanese Patent Application Publication No. 2011-147445) using Frozen-EZ Yeast Transformation II (Zymo Research), which was then cultured in 5 mL of an SD liquid culture medium.

Fourth Embodiment

(Enrichment Culture in Culture Medium Using Xylose as Carbon Source)

The SD culture solution in the Third Embodiment after a 2-day culture was added to 5 mL of an SX liquid culture medium, and cultured at 30 deg C., and 70 rpm in a BioPhotorecorder TVS062CA (Advantech). Cells were recovered from the culture solution on day 7 from the initiation of the culture, and added to 5 mL of a fresh SX liquid culture medium to a initial culture solution concentration of 0.1 in terms of OD (600 nm). The solution was cultured at 30 deg C., and 70 rpm for 7 days using a BioPhotorecorder, and cells were recovered from the culture solution. The cells were spread on an SX agar culture medium and cultured at 30 deg C. Colonies grown faster than yeast, to which a DNA fragment of RsXI-C1-opt was introduced, were selected, streaked on an SD agar culture medium and cultured to be purified as selected strains.

Fifth Embodiment

(Extraction of Plasmids from Selected Strains and Sequencing Analyses)

From top 10 strains in terms of specific growth rate in the growth test in the Fourth Embodiment, and the strain having introduced RsXI-C1-opt, plasmids were extracted using a Yeast Plasmid Minipreparation kit, Zymoprep (Zymo research). As the results of analyses of RsXI-C1-opt gene domains in extracted plasmids, 5 types of mutant sequences were recognized. The mutant XI genes were designated respectively as RsXIC1O-T76I, RsXIC1O-E125G, RsXIC1O-I286F, RsXIC1O-N337T and RsXIC1O-K384E; and yeast expression vectors for the respective genes were designated as pRS316GAP-RsXIC1O-T76I, pRS316GAP-RsXIC1O-E125G, pRS316GAP-RsXIC1O-I286F, pRS316GAP-RsXIC1O-N337T, and pRS316GAP-RsXIC10-K384E. Further, a yeast expression vector of a wild type RsXI-C1-opt was designated as pRS316GAP-RsXIC1O.

Sixth Embodiment

(Introduction of Mutant Gene into Yeast)

A plasmid prepared in the Fifth Embodiment was introduced in yeast of Saccharomyces cerevisiae W600W strain (see Japanese Patent Application Publication No. 2011-147445) identically with the Third Embodiment using Frozen-EZ Yeast Transformation II (Zymo Research), and the yeast was spread on an SD agar culture medium and cultured at 30 deg C. Grown colonies were streaked over a fresh SD agar culture medium and cultured for purification. The obtained selected strains after purification as well as the used plasmids were designated as below.

WR701Is: pRS316GAP-RsXIC1O-T76I

WR702Gs: pRS316GAP-RsXIC1O-E125G

WR703Fs: pRS316GAP-RsXIC1O-I286F

WR704 Ts: pRS316GAP-RsXIC1O-N337T

WR705Es: pRS316GAP-RsXIC10-K384E, and

WR700s: pRS316GAP-RsXIC1O

Seventh Embodiment

(Growth Test of Genetically Modified Yeast Utilizing Xylose as Carbon Source)

Growth tests in a culture medium containing xylose as a carbon source were conducted for evaluating the xylose utilization ability of the yeasts obtained in the Sixth Embodiment. 5 types of the strains prepared in the Sixth Embodiment were cultured in an SD liquid culture medium for 24 hours, and the cells were recovered and washed with sterilized water. Thereafter, in an SX liquid culture medium prepared in an L-shaped test tube, the cells were added and a growth test was initiated. During the growth test under culture conditions of 30 deg C., 70 rpm using a BioPhotorecorder TVSO62CA, the OD (660 nm) of the culture solution was measured at 20 min intervals. A comparison of the specific growth rates of the respective strains is shown in FIG. 5.

As shown in FIG. 5 it was confirmed that the specific growth rates of the WR703Fs strain having introduced RsXIC1O-I286F, the WR704 Ts strain having introduced RsXIC1O-N337T, and the WR705Es strain having introduced RsXIC1O-K384E are higher than that of the WR700s strain having introduced the wild type RsXI-C1-opt. Among others, the specific growth rate of the WR704 Ts strain was 1.6-fold the WR700s strain.

Eighth Embodiment

(Fermentation Test of Genetically Modified Yeast Utilizing Xylose as Carbon Source)

The WR700s strain and the WR704 Ts strain were inoculated in a 5 mL of SD liquid culture medium and cultured for 24 hours. Next, 1 mL of the culture solution was added to 50 mL of an SD liquid culture medium and cultured for 24 hours. The cells were recovered and washed twice by sterilized water. For a fermentation test a pressure-resistant test tube with a screw top sealed tightly with a butyl rubber closure was used. 5 mL of an SX liquid culture medium 50 was prepared by adding a yeast suspension to the final OD (600 nm) of a fermentation medium of 10, and fermented at 30 deg C., and 180 rpm. At discretionary timing an aliquot of the fermented liquid was sampled and analyzed by liquid chromatography about xylose and ethanol. As a column for liquid chromatography an HPX-87H column (Bio-RAD) was used at 60 deg C., and as a detector a differential refractive index detector RID-10A (Shimadzu Corporation) was used. For a mobile phase a 0.05% sulfuric acid solution was used, and supplied at a flow rate of 0.8 mL/min. FIG. 6 shows time-dependent changes of xylose concentration and ethanol concentration in the fermentation medium with respect to the respective strains. In this connection, the fermentation tests were repeated 4 times, and the average values are shown.

As shown in FIG. 6, the consuming rate of xylose and the ethanol production rate of the WR704 Ts strain are approx. 2.5 times as higher as the WR700s strain, and the xylose consumption by fermentation for 72 hours was approx. 12 g/L for the WR700s strain but approx. 30 g/L for the WR704 Ts strain. From the above it has become clear that the xylose utilization ability of yeast can be improved by substituting asparagine at position 337 of RsXI with threonine.

Ninth Embodiment

(Construction of Amino Acid Point Mutation Library and Introduction into Yeast)

An amino acid point mutation library was constructed targeting the 337th amino acid (asparagine) of RsXIC10m, for which an improvement effect of xylose utilization ability in yeast was confirmed. A reaction was conducted using pRS316GAP-RsXIC1O described in the Fifth Embodiment as a template, primers listed in Table 1 below, and a a Quick Change Lightning MultiSite-Directed Mutagenesis kit (Agilent Technologies, Inc.) according to the protocol attached to the kit. Using the obtained reaction solution, ECOS Competent E. coli DH5 alfa (Nippon Gene Co., Ltd.) was transformed and a plasmid was extracted from a grown colony. By sequencing a mutated locus was identified and a plasmid for introduction to yeast for each of mutant XI having any of 18 types of mutations except asparagine and threonine (alanine: A, arginine: R, aspartic acid: D, cysteine: C, glutamine: G, glutamic acid: E, glycine: G, histidine: H, isoleucine: I, leucine: L, lysine: K, methionine: M, phenylalanine: F, proline: P, serine: S, tryptophan: W, tyrosine: Y, and valine: V) was obtained (Table 1). Then an obtained plasmid was introduced in the W600W strain using Frozen-EZ Yeast Transformation II, which was then spread on an SD agar culture medium. A grown colony was streaked over a fresh SD agar culture medium and cultured to purify the colony. The obtained genetically modified yeasts, plasmids used for introducing mutant XI into yeast, and introduced mutant XI genes are shown in Table 1.

TABLE 1 Strain Vector Gene Primer SEQ ID WR704 As pRS316GAP-RsXIO-N337 A RsXIO-N337A RsXI-N337 A-FP-F 45 WR704 Rs pRS316GAP-RsXIO-N337 R RsXIO-N337R RsXI-N337 R-FP-F 46 WR704 Ds pRS316GAP-RsXIO-N337 D RsXIO-N337D RsXI-N337 D-FP-F 47 WR704 Cs pRS316GAP-RsXIO-N337 C RsXIO-N337C RsXI-N337 C-FP-F 48 WR704 Qs pRS316GAP-RsXIO-N337 Q RsXIO-N337Q RsXI-N337 Q-FP-F 49 WR704 Es pRS316GAP-RsXIO-N337 E RsXIO-N337E RsXI-N337 E-FP-F 50 WR704 Gs pRS316GAP-RsXIO-N337 G RsXIO-N337G RsXI-N337 G-FP-F 51 WR704 Hs pRS316GAP-RsXIO-N337 H RsXIO-N337H RsXI-N337 H-FP-F 52 WR704 Is pRS316GAP-RsXIO-N337 I RsXIO-N337I RsXI-N337 I-FP-F 53 WR704 Ls pRS316GAP-RsXIO-N337 L RsXIO-N337L RsXI-N337 L-FP-F 54 WR704 Ks pRS316GAP-RsXIO-N337 K RsXIO-N337K RsXI-N337 K-FP-F 55 WR704 Ms pRS316GAP-RsXIO-N337 M RsXIO-N337M RsXI-N337 M-FP-F 56 WR704 Fs pRS316GAP-RsXIO-N337 F RsXIO-N337F RsXI-N337 F-FP-F 57 WR704 Ps pRS316GAP-RsXIO-N337 P RsXIO-N337P RsXI-N337 P-FP-F 58 WR704 Ss pRS316GAP-RsXIO-N337 S RsXIO-N337S RsXI-N337 S-FP-F 59 WR704 Ws pRS316GAP-RsXIO-N337 W RsXIO-N337W RsXI-N337 W-FP-F 60 WR704 Ys pRS316GAP-RsXIO-N337 Y RsXIO-N337Y RsXI-N337 Y-FP-F 61 WR704 Vs pRS316GAP-RsXIO-N337 V RsXIO-N337V RsXI-N337 V-FP-F 62

Tenth Embodiment

(Fermentation Test of Genetically Modified Yeast Utilizing Xylose as Carbon Source)

18 recombinant yeasts in Table 1, the WR700s strain, and the WR704 Ts strain were inoculated to 1 mL of SD liquid culture media prepared in a 96-well Storage Block (Corning Incorporated) with a volume of 2 mL per each well, and cultured in a constant temperature incubator shaker M-BR-022UP (Taitec Corporation) at 30 deg C., and 1500 rpm, for 24 hours. Next, 200 microliters of the culture solutions were added to 1 mL of SD liquid culture media prepared in a 96-well Storage Block and cultured under the similar conditions for 24 hours. The cells were recovered, washed twice with sterile water, and suspended in sterile water to prepare yeast suspensions. A fermentation test was performed under the following conditions. 1 mL of an SX liquid culture medium was prepared in a 96-well Storage Block, such that a yeast suspension was added therein to the final OD (600 nm) of 1. For establishing an anaerobic condition each well was hermetically sealed with Titer Stick HC (Kajixx Co., Ltd.), and fermentation was conducted in an M-BR-022UP under conditions of 30 deg C., and 1500 rpm. At discretionary timing an aliquot of the fermented liquid was sampled and analyzed by liquid chromatography about xylose and ethanol as in Eighth Embodiment. FIG. 7 shows xylose consumption by yeast having introduced XIs from the initiation of the fermentation to 72 hours there after. The fermentation tests were repeated 2 or more times, and the average values are shown.

As shown in FIG. 7, the xylose consumption after fermentation of 72 hours of the WR700s strain having introduced the wild type RsXI gene (RsXI-C1-opt) was 2.6 g/L, and the xylose consumption of the WR704 Ts strain shown in the Seventh Embodiment was 7.2 g/L. It was confirmed that among 18 types of strains obtained from the point mutation library, the xylose consumption was improved in 3 strains of WR704Cs, WR704Vs, and WR704As surpassing the WR700s (respectively, 8.9 g/L, 5.3 g/L, and 4.0 g/L). Among others, the xylose consumption of the WR704Cs strain exceeded that of the WR704 Ts strain, and it was confirmed that the same was improved to a level 3.5-fold as high as WR700s. From the above it became clear that the xylose utilization ability of yeast could be improved 1.5-fold or more by substituting asparagine at position 337 of RsXI with any one of threonine, cysteine, valine and alanine (DNAs coding for the proteins are expressed respectively by SEQ ID NOs:71 to 74).

Eleventh Embodiment

(Introduction of Amino Acid Point Mutation into Other XIs)

Whether the improvement effect on xylose utilization ability obtained by a mutation with respect to the 337th amino acid of RsXI is reproducible in XIs originated from other biological species was investigated. Using a xylose isomerase gene originated from Piromyces sp. E2, and a xylose isomerase gene originated from Clostridium phytofermentans (Japanese Patent Application Publication No. 2011-147445), which were optimized to yeast codons, as templates, as well as primers listed in the following Table 2, DNA fragments PiXIO-N338T and CpXIO-N337T (SEQ ID NO:75 and SEQ ID NO:76) with introduced mutations substituting asparagine correspond to the 337th amino acid residue of RsXI with threonine, were synthesized. Further, as a control, DNA fragments, PiXIO and CpXIO without an introduced mutation were also simultaneously synthesized. An obtained DNA fragment was inserted into pRS316GAP digested by restriction enzymes SacII and XhoI using an In-Fusion HD PCR cloning kit (Takara Bio Inc.) to obtain a gene transduction plasmid. Then an obtained plasmid was introduced in the W600W strain using Frozen-EZ Yeast Transformation II, which was then spread on an SD agar culture medium. A grown colony was streaked over a fresh SD agar culture medium and cultured to purify the colony. The obtained genetically modified yeasts, plasmids used for introducing a mutant XI into yeast, and introduced mutant XI genes are shown in Table 2.

TABLE 2 Strain Vector Gene Primer SEQ ID WP704Ts pRS316GAP-PiXIO-N338T PiXIO-N338T PiXI-opt-IF-F2 63 PiXI-opt-N338T-FP-R 64 PiXI-opt-N338T-FP-F 65 PiXI-opt-IF-R2 66 WP700s pRS316GAP-PiXIO PiXIO PiXI-opt-IF-F2 63 PiXI-opt-IF-R2 66 WC704Ts pRS316GAP-CpXIO-N337T CpXIO-N337T CpXI-opt-IF-F2 67 CpXI-opt-N337T-FP-R 68 CpXI-opt-N337T-FP-F 69 CpXI-opt-IF-R2 70 WC700s pRS316GAP-CpXIO CpXIO CpXI-opt-IF-F2 67 CpXI-opt-IF-R2 70

Twelfth Embodiment

(Fermentation Test of Genetically Modified Yeast Utilizing Xylose as Carbon Source)

4 types of the recombinant yeasts listed in Table 2 were inoculated in 5 mL of SD liquid culture media, and cultured at 30 deg C., and 100 rpm for 24 hours. In 5 mL of a freshly prepared SD liquid culture medium, 200 microliters of a culture solution was added, and cultured under similar conditions for 24 hours. Cells were recovered, washed twice with sterile water, and suspended in sterile water to prepare a yeast suspension. A fermentation test was performed under the following conditions. 1 mL of an SX liquid culture medium was prepared in a 96-well Storage Block, such that the yeast suspension was added therein to the final OD (600 nm) of 10 in the cases of the WP700s strain and the WP704 Ts strain, and 50 in the cases of the WC700s strain and the WC704 Ts strain. For establishing an anaerobic condition each well was hermetically sealed with Titer Stick HC and fermentation was conducted under conditions of 30 deg C., and 1500 rpm. At discretionary timing an aliquot of the fermented liquid was sampled and analyzed by liquid chromatography about xylose and ethanol similarly as in the Eighth Embodiment. FIG. 8 shows xylose consumption by yeast having introduced XIs from the initiation of the fermentation to 72 hours there after. The fermentation tests were repeated 2 or more times, and the average values are shown.

As shown in FIG. 8A, the xylose consumption after fermentation of 72 hours of a WP700s strain having introduced a XI gene originated from a wild type Piromyces sp. E2 (PiXIO) was 2.4 g/L, but the xylose consumption of the WP704 Ts strain having introduced a mutated type XI gene (PiXIO-N338T) was 7.3 g/L. From the above it was confirmed that the xylose consumption of the WP704 Ts strain was improved 3.1-fold compared to the WP700s strain.

Further, as shown in FIG. 8B, the xylose consumption after fermentation of 72 hours of the WC700s strain having introduced a XI gene originated from a wild type Clostridium phytofermentans (CpXIO) was 1.8 g/L, but the xylose consumption of a WC704 Ts strain having introduced a mutated type XI gene (CpXIO-N337T) was 2.2 g/L. From the above it was confirmed that the xylose consumption of the WC704 Ts strain was improved 1.2-fold compared to the WC700s strain. From the above it has become clear that the xylose utilization ability of yeast can be improved with PiXI or CpXI similarly as with RsXI by introducing a mutation to the position correspond to position 337 of RsXI.

Thirteenth Embodiment

(Introduction of Mutation into XIs Originated from Other Biological Species)

With respect to a XI originated from Piromyces sp. E2 (PiXI), a XI originated from Clostridium phytofermentans (CpXI), a XI originated from Bacteroides thetaiotaomicron (BtXI), and a XI originated from Lactococcus lactis (LlXI), for which the activities in a yeast were reported, it was investigated whether the improvement effect on xylose utilization ability could be obtained by substituting an amino acid corresponding to asparagine in position 337 of RsXI with alanine, cysteine, threonine, or valine. Table 3 shows strains, plasmids, genes and primers used for transducing mutations, and Table 4 shows the primer sequences.

TABLE 3 Strain Plasmid Gene Primer Seq ID WP700s pRS316GAP-PiXIO PiXIO PiXI-opt-IF-F2 77 PiXI-opt-IF-R2 78 WP704As pRS316GAP-PiXIO-N338A PiXIO-N338A PiXIO-N338A-FP-F 86 WP704Cs pRS316GAP-PiXIO-N338C PiXIO-N338C PiXIO-N338C-FP-F 87 WP704Ts pRS316GAP-PiXIO-N338T PiXIO-N338T PiXIO-N338T-FP-F 88 WP704Vs pRS316GAP-PiXIO-N338V PiXIO-N338V PiXIO-N338V-FP-F 89 WC700s pRS316GAP-CpXIO CpXIO CpXI-opt-IF-F2 79 CpXI-opt-IF-R2 80 WC704As pRS316GAP-CpXIO-N337A CpXIO-N337A CpXIO-N337A-FP-F 90 WC704Cs pRS316GAP-CpXIO-N337C CpXIO-N337C CpXIO-N337C-FP-F 91 WC704Ts pRS316GAP-CpXIO-N337T CpXIO-N337T CpXIO-N337T-FP-F 92 WC704Vs pRS316GAP-CpXIO-N337V CpXIO-N337V CpXIO-N337V-FP-F 93 WB700s pRS316GAP-BtXI BtXI BtXI-IF-F 81 BtXI-IF-B 82 WB704As pRS316GAP-BtXI-N339A BtXI-N339A BtXI-N339A-FP-F 94 WB704Cs pRS316GAP-BtXI-N339C BtXI-N339C BtXI-N339C-FP-F 95 WB704Ts pRS316GAP-BtXI-N339T BtXI-N339T BtXI-N339T-FP-F 96 WB704Vs pRS316GAP-BtXI-N339V BtXI-N339V BtXI-N339V-FP-F 97 WL700s pRS316GAP-LIXIO LIXIO LIXI-opt-IF-F 84 LIXI-opt-IF-R 85 WL704As pRS316GAP-LIXIO-N337A LIXIO-N337A LIXIO-N337A-FP-F 98 WL704Cs pRS316GAP-LIXIO-N337C LIXIO-N337C LIXIO-N337C-FP-F 99 WL704Ts pRS316GAP-LIXIO-N337T LIXIO-N337T LIXIO-N337T-FP-F 100 WL704Vs pRS316GAP-LIXIO-N337V LIXIO-N337V LIXIO-N337V-FP-F 101

TABLE 4 Seq ID Primer name Sequence  77 PiXI-optIF-F2 5′-ataaacaaacaaaccg cggaaaatggctaaggaat acttcccacaaatccaaaa gattaaattcgaggg-3′  78 PiXI-optIF-R2 5′-tgatgcggccctcgag ttattggtacatagcaaca attgcttcatacaattctt gtttaccac-3′  86 PiXIO-N338A-FP-F 5′-atcagaggtggtggtt ttgttacaggtggtaccgc tttcgatgcaaaaacca g-3′  87 PiXIO-N338C-FP-F 5′-atcagaggtggtggtt ttgttacaggtggtacctg tttcgatgcaaaaacca g-3′  88 PiXIO-N338T-FP-F 5′-atcagaggtggtggtt ttgttacaggtggtaccac tttcgatgcaaaaacca g-3′  89 PiXIO-N338V-FP-F 5′-atcagaggtggtggtt ttgttacaggtggtaccgt tttcgatgcaaaaacca g-3′  79 CpXI-opt-IF-F2 5′-ataaacaaacaaaccg cggaaaatgaagaattact tcccaaatgtcccagaagt gaaatatgaaggccc-3′  80 CpXI-opt-IF-R2 5′-tgatgcggccctcgag tcatctaaacaagatgtta ttgacaatagtctccaaga cttcttgtc-3′  90 CpIXO-N337A-FP-F 5′-tgaaagctggaggctt tactaatggtggtctagct tttgatgctaaggttagaa gaggcag-3′  91 CpIXO-N337C-FP-F 5′-tgaaagctggaggctt tactaatggtggtctatgt tttgatgctaaggttagaa gaggcag-3′  92 CpIXO-N337T-FP-F 5′-tgaaagctggaggctt tactaatggtggtctaact tttgatgctaaggttagaa gaggcag-3′  93 CpIXO-N337V-FP-F 5′-tgaaagctggaggctt tactaatggtggtctagtt tttgatgctaaggttagaa gaggcag-3′  81 BtXI-IF-F 5′-ataaacaaacaaaccg cggaaaatggcaacaaaag aattttttccgggaattga aaagattaaatttg-3′  82 BtXI-IF-R 5′-tgatgcggccctcgag ttaatacatattcagaatt gcctcataaagttcttgct tgc-3′  94 BtXI-N339A-FP-F 5′-cggtaccggtggtacg gcttttgatgctaaaaccc gtcgtaattctactgat c-3′  95 BtXI-N339C-FP-F 5′-cggtaccggtggtacg tgttttgatgctaaaaccc gtcgtaattctactgat c-3′  96 BtXI-N339T-FP-F 5′-cggtaccggtggtacg acttttgatgctaaaaccc gtcgtaattctactgat c-3′  97 BtXI-N339V-FP-F 5′-cggtaccggtggtacg gtttttgatgctaaaaccc gtcgtaattctactgat c-3′  84 LIXI-opt-IF-F 5′-ataaacaaacaaaccg cggaaaatggcctacttta acgacatcgcaccaatcaa atacgaaggtactaag-3′  85 LIXI-opt-IF-R 5′-tgatgcggccctcgag ttataccaagtagtcgttc aaaacactctttatgtatt ccaaatgg-3′  98 LIXIO-N337A-FP-F 5′-gaacggtggtttgggt aaaggtggtatagcttttg atgccaaagtcagaagaac atc-3′  99 LIXIO-N337C-FP-F 5′-gaacggtggtttgggt aaaggtggtatatgttttg atgccaaagtcagaagaac atc-3′ 100 LIXIO-N337T-FP-F 5′-gaacggtggtttgggt aaaggtggtataacttttg atgccaaagtcagaagaac atc-3′ 101 LIXIO-N337V-FP-F 5′-gaacggtggtttgggt aaaggtggtatagtttttg atgccaaagtcagaagaac atc-3′

(Preparation of Template DNA)

Preparation of a template DNA for introducing a point mutation was performed as follows. With respect to PiXI and CpXI, the respective DNA fragments were synthesized using a xylose isomerase gene originated from Piromyces sp. E2 and a xylose isomerase gene originated from Clostridium phytofermentans, in which codons were optimized for expression in yeast, as templates, as well as primers listed in Table 4 (SEQ ID NOs:77, 78, 79 and 80). The obtained DNA fragments were inserted in pRS316GAP digested by restriction enzymes SacII and XhoI using an In-Fusion HD PCR cloning kit to construct pRS316GAP-PiXIO, and pRS316GAP-CpXIO (Table 3).

With respect to BtXI, the DNA fragment was synthesized using a genome DNA (ATCC 29148D) originated from B. thetaiotaomicron VPI 5482 furnished by ATCC (American Type Culture Collection) as a template. The used primers (SEQ ID NOs:81, 82) are listed in Table 4. The obtained DNA fragment was inserted in pRS316GAP digested by restriction enzymes SacII and XhoI using an In-Fusion HD PCR cloning kit to construct pRS316GAP-BtXI (Table 3).

With respect to LlXI, a synthetic gene LlXIO (SEQ ID NO:83), in which codons were optimized for expression in yeast, was synthesized (Genscript Corporation (www.Genscript.com)) based on an amino acid sequence described in Patent Literature 3 and an amino acid sequence acquired from Genbank (Genbank: AAD20249). Next, a DNA fragment was synthesized using the prepared LlXIO as a template, and primers listed in Table 4 (SEQ ID NOs:84, 85). The obtained DNA fragment was inserted in pRS316GAP digested by restriction enzymes SacII and XhoI using an In-Fusion HD PCR cloning kit to construct pRS316GAP-LlXIO (Table 3).

Using pRS316GAP-PiXIO, pRS316GAP-CpXIO, pRS316GAP-LlXIO and pRS316GAP-BtXI as a template, primers (SEQ ID NOs:86 to 101) listed in Table 4, and a QuickChange Lightning Multi Site-Directed Mutagenesis kit (Agilent Technologies, Inc.), a reaction was carried out according to a protocol attached to the kit. Using the obtained reaction solution, transformation of ECOS (Trademark) Competent E. coli DH5 alfa (Nippon Gene Co., Ltd.) was conducted, and plasmids were extracted from grown colonies. By sequencing a mutated locus was identified, and for each XI, plasmids for transducing 4 variant XI genes, in which asparagine was substituted with cysteine, threonine, valine, and alanine, were obtained (Table 3).

Then, the obtained plasmid was introduced in the W600W strain using Frozen-EZ Yeast Transformation II, and spread on an SD agar culture medium. A grown colony was streaked over a fresh SD agar culture medium and cultured to purify the colony.

Fourteenth Embodiment

(Fermentation Test of Genetically Modified Yeast Utilizing Xylose as Carbon Source)

In 1 mL of SD liquid culture media prepared in a 96-well Storage Block, 20 types of recombinant yeasts listed in Table 3 were inoculated and cultured in a constant temperature incubator shaker M-BR-022UP at 30 deg C., and 1500 rpm, for 24 hours. Then 200 microliters of a culture solution was added in 1 mL of an SD liquid culture medium prepared in a fresh 96-well Storage Block and culture under similar conditions for 24 hours. Cells were recovered, washed twice with sterile water, and then suspended in sterile water to prepare a yeast suspension.

A fermentation test was conducted under the following conditions. 1 mL of an SX liquid culture medium (6.7 g/L of Yeast Nitrogen Base without amino acids, and 20 g/L of xylose) was prepared in a 96-well Storage Block, in which the yeast suspension was added such that the final OD₆₀₀ of the culture medium became 10. Then, fermentation was carried out as in the Twelfth Embodiment, and xylose and ethanol were analyzed by liquid chromatography. FIG. 9 shows xylose consumptions by yeasts having introduced various XI genes from the initiation of the fermentation to 72 hours thereafter. The fermentation tests were repeated 2 or more times, and the average values are shown.

As shown in FIG. 9A, the xylose consumption after fermentation of 72 hours of the WP700s(WT) strain was 2.8 g/L, however in contrast thereto the xylose consumptions of the WP704Cs(C), the WP704 Ts(T) strain, and the WP704Vs(V) strain having introduced a mutated type XI gene, in which asparagine was substituted with cysteine, threonine, or valine, were respectively 5.1 g/L, 4.1 g/L, and 3.9 g/L to confirm improvement of the xylose utilization ability. Similarly with respect to other XIs, improvement of the xylose utilization ability was confirmed for strains having introduced mutated type XI genes, in which asparagine was substituted with cysteine or valine in the case of CpXI (FIG. 9A), cysteine or threonine in the case of BtXI (FIG. 9C), and cysteine, threonine, or alanine in the case of LlXI (FIG. 9D).

As the result of the above, it became clear that similar to RsXI also in the case of XIs originated from other organisms, such as PiXI, CpXI, BtXI, and LlXI, the xylose utilization ability of yeast could be improved by introducing a mutation at a locus corresponding to asparagine at position 337 in RsXI.

[Sequence Listing Free Text]

SEQ ID NOs: 2-13, 201-105: consensus sequence in Xylose isomerase

SEQ ID NOs: 30-40: xylose isomerase mutant

SEQ ID NOs: 41-70, 77-101: primer

SEQ ID NOs: 71-76: xylose isomerase mutant 

The invention claimed is:
 1. A DNA coding for a protein that has xylose isomerase activity and has an amino acid sequence including, when aligned with an amino acid sequence expressed by SEQ ID NO:1, the following 1st to 6th motifs from the N-terminus of the protein in the order described, and having, in place of asparagine (N) in an amino acid sequence of the 6th motif, an amino acid selected from the group consisting of cysteine, threonine, valine, and alanine: 1st motif: FXXXXKXXXXXXXXHDXD (SEQ ID NO:2) wherein X represents a naturally occurring amino acid, 2nd motif: XXXXXXXWGGREGYXXLXNT (SEQ ID NO:3) wherein X represents a naturally occurring amino acid, 3rd motif: XXXXXXXXEPKPXEPXXHQYDXD (SEQ ID NO:4) wherein X represents a naturally occurring amino acid, 4th motif: LXXXXXXNXEXNHXXLXXHXXXH (SEQ ID NO:5) wherein X represents a naturally occurring amino acid, 5th motif: XGSXDXNXGXXXXGWDXDXXP (SEQ ID NO:6) wherein X represents a naturally occurring amino acid, and 6th motif: GGXNFDXKXRR (SEQ ID NO:7) wherein X represents a naturally occurring amino acid.
 2. A transformation vector for a eukaryotic cell, containing the DNA according to claim
 1. 3. A eukaryotic cell retaining the DNA according to claim
 1. 4. The eukaryotic cell according to claim 3, which is yeast.
 5. The eukaryotic cell according to claim 4, wherein the yeast belongs to any one selected from the group consisting of Saccharomyces, Kluyveromyces, Candida, Pichia, Schizosaccharomyces, Hancenula, Klocckera, Schwanniomyces, Yarrowia, and Issatchenkia.
 6. The eukaryotic cell according to claim 3, which produces secretory cellulase.
 7. The eukaryotic cell according to claim 3, having an exogenous or endogenous gene that produces any one selected from the group consisting of ethanol, lactic acid, acetic acid, 1,3-propanediol, propanol, butanol, succinic acid, ethylene, glycerol, farnesol, geranylgeraniol, and squalene.
 8. A method for generating a eukaryotic cell with imparted or improved xylose utilization ability, comprising a step of introducing the DNA according to claim 1 into a eukaryotic cell for transformation.
 9. A method comprising culturing the eukaryotic cell according to claim 3 in the presence of xylose to form a useful substance.
 10. The method according to claim 9, wherein the useful substance is any one selected from the group consisting of ethanol, lactic acid, acetic acid, 1,3-propane diol, propanol, butanol, succinic acid, ethylene, glycerol, farnecol, geranyl geraniol and squalene. 